windows/msvc: Make mpy-cross independent of micropython variant.

The variant.props may have incompatible build options which break
the mpy-cross build and in any case mpy-cross has nothing to do
with variant support.

ports/windows/msvc/common.props

@@ -3,14 +3,14 @@
   <!-- Variant support. For compatibility with how it works for the other ports, this gets imported
        early so variants cannot override build options like the ones specified in the rest of this file.
        Use CustomPropsFile (see the .vcxproj file) for that. -->
-  <PropertyGroup>
+  <PropertyGroup Condition="'$(PyBuildingMpyCross)' != 'True'">
     <PyVariant Condition="'$(PyVariant)' == ''">standard</PyVariant>
     <PyBuild Condition="'$(PyBuild)' == ''">build-$(PyVariant)</PyBuild>
     <PyProg Condition="'$(PyProg)' == ''">micropython</PyProg>
   </PropertyGroup>
   <ImportGroup Label="PropertySheets">
     <Import Project="paths.props" Condition="'$(PyPathsIncluded)' != 'True'"/>
-    <Import Project="$(PyVariantDir)mpconfigvariant.props"/>
+    <Import Condition="'$(PyBuildingMpyCross)' != 'True'" Project="$(PyVariantDir)mpconfigvariant.props"/>
   </ImportGroup>
   <PropertyGroup Label="UserMacros" />
   <PropertyGroup>